1. Field of the Invention
This invention relates to linear scale type measuring instruments, and particularly to improvements in a linear scale type measuring instrument comprising a hollow case, a linear scale housed in the hollow case and a slider movable along the linear scale.
2. Description of the Prior Art
One type of length measuring apparatus for measuring or adjusting relative positions of two objects in the linear scale type measuring instrument. This linear scale type measuring instrument is constructed as shown in FIGS. 1 through 3 for example. In the drawing: designated at 10 is a tubular, hollow aluminum case being substantially rectangular in cross section, formed by cold drawings for example, and fixed one of the members, relative positions of which are to be measured; 12 a glass linear scale received at the lower end thereof in a longitudinal groove 10a formed in the hollow case 10, and elastically bonded to the hollow case 10 through a rubber bar 14 and an elastomeric bonding agent 16 such as silicon rubber; 20 a detecting section fixed to the other member, the relative position of which is to be measured; 22 a slider connected to an arm 20a formed at the lower portion of the detecting section 20 through connecting means comprising a cantilever spring 24, one end of which is fixed to the arm 20a and the other end of which is coupled onto a conical projection 23 for example, and movable along the linear scale 12 in a condition of being biased by the cantilever spring 24 in a direction of being brought into abutting contact with the surface of the linear scale 12;26 projector elements arranged on the slider 22;28 an index scale fixed to the slider 22;30 receptor elements arranged on the slider at positions opposite to the aforesaid projector elements 26 for receiving rays emitted from the projector elements and passed through a graduated scale 12a of the linear scale 12 and a graduated scale of the index scale; 32, 33 sliding blocks constituting a travel guide mechanism for holding a predetermined interval between a graduated surface 12b or an end face of the linear scale serving as a scanning reference surface for the slider 22 and the slider 22; 34 a belt-like steel tape for preventing dust and the like from entering into the hollow case 10 from outside; and 36 magnets embedded in the hollow case 10 for attracting the steel tape 34.
In the conventional linear scale type measuring instrument of the type described, when a relative movement in a direction perpendicular to the paper surface in FIG. 1 is effected between the hollow case 10 fixed to one of the objects moving relatively with each other and the detecting section fixed to the other of the objects; the amount of rays transmitted through the graduated scales of the index scale 28 and of the linear scale 12, which have been emitted from the projector elements 26, are varied, whereby the variations in amount of transmitting rays are detected by the receptor elements 30, so that a relative movement between the hollow case 10 and the detecting section 20 can be measured.
With the linear scale type measuring instrument of the type described, there is such a characteristic feature that the relative displacement value between two objects can be very accurately measured. However, in the conventional linear scale type measuring instrument, the graduated surface 12b of the linear scale 12 and an end face 12c of the linear scale perpendicular to the graduated surface 12b have been used as the scanning reference surface for the slider 22, and hence, in order to improve the degree of accuracy in measurement, it has been necessary to make with a high degree of straightness not only the graduated surface 12b which is finely finished in general but also on the end face 12c of the linear scale, whereby the end face 12c of the linear scale is brought into parallel to the graduated scale 12a accurately. However, as a matter of fact, it is difficult to accurately finish the end face 12c of the linear scale 12, and, even if it is possible to do so, it entails an increase in manufacturing costs to a considerable extent. Particularly, in the case the end face 12c of the linear scale 12 is to be engaged with the slider 22 through the sliding blocks 33 as in the above described example of the prior art, not only straightness but also very fine surface roughness are required from the end face 12c of the linear scale, thus resulting in increased cost to a greater extent.
On the other hand, such a measuring instrument may be proposed that, in which, one of the scanning reference surfaces for the slider 22 is served by the graduated surface 12b of the linear scale 12 similarly to the prior art and the other of the scanning reference surfaces is served by the inner surface 10 of the hollow case 10. However, the hollow case 10 is made of an aluminum material formed by drawing, and hence, sweeps of the hollow case tend to occur in the course of manufacture and there are possibilities of deformations caused to the hollow case due to wear, thus presenting such a disadvantage that it is difficult to achieve a required degree of accuracy in measurement for a long period of time.